As energy prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, the demand for environmentally-friendly alternative energy sources is bound to play an increasing role in the future. Thus, research into techniques for generating various powers, such as nuclear energy, solar energy, wind energy, and tidal power, is underway, and power storage apparatuses for more efficient use of the generated energy are also drawing much attention.
In particular, the demand for batteries as energy sources is rapidly increasing as mobile device technology continues to develop and the demand for the mobile devices continues to increase. Accordingly, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is very high.
In addition, secondary batteries may be classified based on the structure of an electrode assembly having a structure in which a positive electrode and a negative electrode are stacked in a state in which a separator is interposed between the positive electrode and the negative electrode. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type positive electrode and a long sheet type negative electrode are wound in a state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type structure in which pluralities of positive electrodes and negative electrodes each having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes. In recent years, in order to solve problems caused by the jelly-roll type electrode assembly and the stacked type electrode assembly, there has been developed a stacked/folded type electrode assembly, which is a combination of the jelly roll type electrode assembly and the stacked type electrode assembly, having an improved structure in which predetermined numbers of positive electrodes and negative electrodes are sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a unit cell, and then a plurality of unit cells is sequentially folded while being placed on a separation film.
In addition, secondary batteries may be classified into a cylindrical battery configured to have a structure in which an electrode assembly is mounted in a cylindrical metal container, a prismatic battery configured to have a structure in which an electrode assembly is mounted in a prismatic metal container, and a pouch-shaped battery configured to have a structure in which an electrode assembly is mounted in a pouch-shaped case made of an aluminum laminate sheet based on the shape of the battery case of each of the secondary batteries.
Particularly, in recent years, much interest has been taken in a pouch-shaped battery configured to have a structure in which such a stacked or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of an aluminum laminate sheet because of low manufacturing costs, light weight, easy modification in shape, etc. In addition, the use of such a pouch-shaped battery has gradually increased.
FIG. 1 is an exploded perspective view typically showing a general structure of a conventional representative pouch-shaped secondary battery.
Referring to FIG. 1, a pouch-shaped secondary battery 100 includes an electrode assembly 130, electrode tabs 131 and 132 extending from the electrode assembly 130, electrode leads 140 and 141 connected respectively to the electrode tabs 131 and 132 by welding, and a battery case 120 for receiving the electrode assembly 130.
The electrode assembly 130 is a power generating element including positive electrodes and negative electrodes sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes. The electrode assembly 130 is configured to have a stacked type structure or a stacked/folded type structure. The electrode tabs 131 and 132 extend from corresponding electrode plates of the electrode assembly 130. The electrode leads 140 and 141 are electrically connected to the electrode tabs 131 and 132, extending from the corresponding electrode plates of the electrode assembly 130, respectively, for example, by welding. The electrode leads 140 and 141 are partially exposed outward from the battery case 120. In addition, insulating films 150 for improving sealability between the battery case 120 and the electrode leads 140 and 141 and, at the same time, securing electrical insulation between the battery case 120 and the electrode leads 140 and 141 are partially attached to the upper and lower surfaces of the electrode leads 140 and 141.
The battery case 120 includes a case body 122 having a concave receiving part 123, in which the electrode assembly 130 is mounted, and a cover 121 integrally connected to the case body 122. In a state in which the electrode assembly 130 is mounted in the receiving part 123, opposite sides 124 and upper ends 125 of the case body 122 and the cover 121, at which the case body 122 and the cover 121 contact each other, are coupled to the each other, whereby the battery is completed. The battery case 120 is configured to have an aluminum laminate structure of a resin layer/metal foil layer/resin layer. Consequently, it is possible to bond the opposite sides 124 and the upper ends 125 of the case body 122 and the cover 121, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 124 and the upper ends 125 of the case body 122 and the cover 121 so as to bond the resin layers thereof to each other, whereby sealed surplus portions are formed. At the opposite sides 124 of the case body 122 and the cover 121, the resin layers of the upper and lower parts of the battery case 120 are directly attached to each other, whereby uniform sealing is achieved by bonding the opposite sides 124 of the case body 122 and the cover 121. On the other hand, the electrode leads 140 and 141 are located at the upper ends 125 of the case body 122 and the cover 121, while protruding outward from the battery case 120. For this reason, the upper ends 125 of the case body 122 and the cover 121 are thermally bonded in a state in which insulation films 150 are interposed between the electrode leads 140 and 141 and the battery case 120 in order to improve sealability considering the thickness of the electrode leads 140 and 141 and the difference in material between the electrode leads 140 and 141 and the battery case 120.
The pouch-shaped secondary battery has an advantage in that the shape of the battery case can be easily modified depending upon the shape of the electrode assembly. However, the pouch-shaped secondary battery has problems in that the secondary battery may not be sufficiently protected when external impart is applied thereto due to low rigidity thereof, and, in particular, the secondary battery may be easily damaged due to a needle-shaped member, whereby the secondary battery may deteriorate and explode.
In addition, when a short circuit may occur in the secondary battery due to the exposure of the secondary battery to a high-temperature environment or the malfunction of the secondary battery, an electrolyte may be decomposed at positive electrode interfaces, whereby a large amount of gas is generated. As a result, the secondary battery may swell, which may damage the electrical connection of the secondary battery.
Furthermore, the pouch-shaped secondary battery has another problem in that the capacity of the secondary battery is reduced due to the sealed surplus portions formed during sealing of the battery case.
In order to solve the above problem, a prismatic battery cell configured having a structure in which a stacked type or stacked/folded type electrode assembly is mounted in a prismatic metal battery case has been considered as a substitute in recent years.
In general, the prismatic battery cell is manufactured by inserting an electrode assembly into a prismatic hollow case having a closed lower end, coupling a top cap assembly to the case by welding, injecting an electrolyte into the case through an injection hole, and sealing the injection hole. The prismatic hollow case having the closed lower end is generally manufactured by deep drawing of an aluminum alloy sheet.
However, deep drawing has problems in that costs necessary to manufacture an apparatus for deep drawing are very high, it takes long time to manufacture a mold, etc., and obtainable shapes are limited.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.